The present invention is directed to an ion-beam monitor and more specifically to an optical observation system for determining the distribution of ion-beam intensity.
A micro-machining process called "ion-beam milling" is generally known in the art and is used in semiconductor fabrication and other micro-lithography processes for removing materials. Typically, a milling machine of this type directs an ion beam towards a target marked by photo-resist material for the purpose of removing material in openings in the photo-resist. The workpiece is generally placed in a vacuum chamber filled with an inactive gas such as argon, and according to such a process, no side etching takes place, that is, no etching takes place beneath the photo-resist pattern. Therefore, the pattern machining accuracy with such a process is on a very high order. The maximum diameter of an ion beam of this type is approximately six inches, and the workpiece is generally in the form of a wafer having a substantially smaller diameter such as three to four inches. In such an ion-beam milling process, it is necessary that the ion beam is of uniform intensity at least within the range of the wafer. Prior to performing an ion-beam milling process, distribution of the beam intensity must be measured, and such conditions as ion beam focusing and the like must be adjusted in order to obtain a uniform distribution of ion beam intensity in order to enhance the accuracy and uniformity of the milling process. However, no effective process is known in the prior art for measuring the distribution of beam intensity with any degree of accuracy.
According to the prior art, attempts were made to measure ion beam intensity within a vacuum chamber filled with inactive gasses such as argon or the like. One method involved irradiating ion beams against a zinc plate to observe the green light emitted therefrom, and another method involved the measuring of the charge of the ion beams. In ion-beam milling, however, a thermionic emission device called a neutralizer is operated and therefore the foregoing methods are insufficient for measuring the ion beam intensity. This is due primarily to the fact that a visible light is present caused by the black body radiation from the neutralizer thereby rendering the measurement of the emission of light from the zinc plate virtually impossible. Furthermore, the charge of the beam is neutralized by the neutralizer and cannot be measured.
Another prior art method of monitoring ion-beam distribution is to melt a uniform layer and note the different resultant thicknesses. However, such a trial and error method has proved to be time consuming and inexact.